Natural Selection

Natural selection is one key driver of evolution. It describes how the environment 'filters' individuals of a species, depending on their genes. Some individuals will be more successful at survival and reproduction than others. Over time, the genes of the most successful individuals become more widespread in a population.

Natural selection can only take place if there is genetic variation between individuals in a population. Non-genetic variation can be caused by lifestyle factors, such as an individual being taller due to better nutrition. This type of variation cannot be passed on to offspring.

Whether a genetic trait is favorable or not depends on the environmental conditions at the time. The same traits are not always selected for, because environmental conditions can change. For example, consider a species of plant that grew in a moist climate and did not need to conserve water. Individuals that possessed gene alleles for the largest leaves would be the most successful, because they could obtain more energy from the sun. Large leaves require more water to maintain than small leaves, but the moist environment provided favorable conditions to support large leaves. After thousands of years, the climate changed, and the area no longer had excess water. The direction of natural selection shifted so that plants with small leaves were selected for, because those populations were able to conserve water to survive in the new environmental conditions.

Natural selection acts on individual organisms, which in turn can shape an entire species. Although natural selection may work in a single generation on an individual, it can take thousands or even millions of years for the genotype of an entire species to evolve. It is over these large time spans that life on earth has changed and continues to change.